A switch mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Power converters are frequently employed to power loads having tight regulation characteristics such as a microprocessor with, for instance, five volts provided from a source of electrical power (e.g., a voltage source). To provide the voltage conversion and regulation functions, the power converters include active switches such as MOSFETs that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of 100 kHz to 10 MHz.
Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage) of the power converter and, based thereon, modifies a duty cycle of the switches (e.g., power switches) of the power converter. The duty cycle is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the needs for systems such as a microprocessor powered by the power converter dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain the output characteristic at a desired value.
In a typical controller for a power converter, the duty cycle is produced at a switching frequency by comparing the amplitude of a threshold voltage controlled by the feedback loop to the amplitude of a sawtooth voltage (or “sawtooth voltage waveform”). When the amplitude of the threshold voltage exceeds the amplitude of the sawtooth voltage, a power switch such as a main power switch is enabled to conduct. When the amplitude of the sawtooth voltage exceeds the amplitude of the threshold voltage, the main power switch is disabled to conduct, and another power switch such as an auxiliary power switch is typically enabled to conduct.
A controller for a power converter is typically formed as an integrated circuit, and its parameters such as the frequency and amplitude of a sawtooth voltage are not tightly controlled during circuit fabrication. When two or more power converters are used in a power conversion application, the different frequencies of operation of the converters result in beat frequencies that may be detrimental to the application at hand. In situations where the beat frequencies are undesirable, it is often necessary to synchronize the switching frequencies of the multiple power converters.
Using currently available techniques, synchronization of the switching frequencies of multiple power converters to a frequency source is readily performed in “current-mode” controllers, but not in “voltage-mode” controllers. Current-mode controllers generate a sawtooth waveform (or “sawtooth ramp”) by sensing the current in a reactive circuit element such as an output filter inductor, or in the inductor's reflected current through a power transformer to a power switch, to provide feedback compensation for the slope of the sawtooth ramp. If the switching frequency is externally synchronized, the slope of the sawtooth ramp generally does not have constant amplitude, which, in the case of current-mode control, may have only a limited effect on the stability of the feedback loop including the controller. A limited range of the amplitude of the sawtooth waveform is generally acceptable in the design of current-mode controllers without substantial adverse effect on the stability of the feedback loop.
Usually the frequency of a frequency source is limited to less than 120% of the free-running frequency of an internal sawtooth waveform oscillator to limit the variation in amplitude of the sawtooth waveform. Otherwise, a proportional decrease in the amplitude of the sawtooth ramp increases the feedback gain of the controller, which may introduce a problem for feedback loop stability, as described in Texas Instruments datasheet TPS40100, entitled “Midrange Input Synchronous Buck Controller with Advanced Sequencing and Output Margining,” Texas Instruments, Inc., Dallas, Tex. (2005) and in Linear Technology datasheet LTC3418, entitled “8A, 4 MHz Monolithic Synchronous Step-Down Regulator,” Linear Technology Corporation, Milpitas, Calif. (2005), which are incorporated herein by reference.
Voltage-mode controllers produce a sawtooth waveform by generating a periodic voltage ramp that is substantially independent of compensating currents in a power train of a power converter. The result is a controller wherein the slope of the sawtooth voltage is independently generated, and that results in a waveform with amplitude proportional to the switching frequency, which, as described above, may be controlled by a separate frequency source. Particularly for voltage-mode controllers synchronized to a separate frequency source, it is often necessary to provide a sawtooth voltage with constant amplitude, independent of the switching frequency, to provide adequate margin for stability of the feedback loop and to preserve feedback-controlled performance characteristics of the power converter such as response time, gain and phase margins, and voltage overshoot for step changes in load or in output voltage.
Another technique to synchronize switching frequencies among multiple power converters is to use a phase-locked-loop. However, phase-locked-loops require a loop filter that is not easily integrated into an ordinary integrated circuit, which adds cost to the implementation of a controller as described in Linear Technology datasheet LTC3736-1, entitled “Dual 2-Phase, No Rsense™, Synchronous Controller with Spread Spectrum,” Linear Technology Corporation, Milpitas, Calif. (2004), which is incorporated herein by reference.
Thus, as discussed above, to preserve tightly controlled feedback-controlled performance characteristics of a power converter, it is necessary to construct a sawtooth generator controlled by a separate frequency source, such as a clock, that produces a sawtooth voltage with a substantially constant amplitude independent of the frequency of a frequency source, using techniques that are easily integrable with an integrated circuit. Sawtooth generators of the prior art that produce a waveform with constant amplitude and are synchronizable to the frequency of a frequency source add measurable cost to a design of a power converter beyond the waveform sourcing functionality thereof.
Accordingly, what is needed in the art is a controller including a sawtooth generator, synchronizable to a separate frequency source, that can produce a sawtooth voltage with amplitude substantially independent of the frequency of the frequency source, that overcomes the deficiencies in the prior art.